Method for preparing particles of metal oxide (tin oxide)

ABSTRACT

PCT No. PCT/EP92/02137 Sec. 371 Date Mar. 23, 1994 Sec. 102(e) Date Mar. 23, 1994 PCT Filed Sep. 16, 1992 PCT Pub. No. WO93/06040 PCT Pub. Date Apr. 1, 1993.A fine metal oxide powder is prepared by a method comprising the steps of (1) preparing a hydroxide precursor of a metal oxide, (2) mixing the precursor with an inorganic compound having a melting point lower than the crystallization temperature of the metal oxide, and (3) subjecting the resulting mixture to a high temperature thermal treatment to form the fine metal oxide powder.

The present invention relates to a method for obtaining particles ofmetal oxide; the invention relates also to the resulting oxide particlesas veil as their use as antistatic agent, particularly for photographicfilms.

The metal oxide powders have various applications, particularly in thefield of antistatic or conductive compositions.

A technique well known to prepare these oxides consists in precipitatinga hydroxide from a salt, and then recovering this hydroxide andsubmitting it to a calcination treatment at a high temperature, higherthan 500° C. and even 1000° C. During this calcination treatment,various phenomena can occur which modify the texture or the crystallinemorphology of the metal oxide particles, and produce a sintering, i.e.,a clustering of these particles. As a result, materials in the form ofparticles exhibiting irregular sizes are obtained. With these particles,it is difficult to prepare dispersions and coating compositions in orderto obtain thin conductive or antistatic layers. Moreover, the physicalproperties of such layers are not very reproducible. Therefore, amechanical treatment such as grinding, is necessary to obtain finerparticles and more homogeneous powders, in order to render thesematerials suitable for the applications considered. However, grindingcauses crystalline defects to appear in the particles, which defectsmust be often resorbed by means of an additional thermal treatment.

In some cases, the hydroxide precipitation is a coprecipitation, becauseeither a second metal hydroxide or an insoluble salt of another metal iscoprecipitated with the hydroxide. An example is the initial step forpreparing hexaferrites of alkaline-earth metals, where a ferrichydroxide and an alkaline-earth metal carbonate are coprecipitated. Thisprecipitate is then thermally processed at about 1000° C. in order toobtain the hexaferrite. In order to promote the reaction betweenhydroxide and carbonate, it is known to add a promoter which can be analkali metal borate, PbO, Bi₂ O₃, an alkali metal ferrite, a molybdate,an alkali metal chloride or sulfate. This method which consists incalcinating the hydroxide/carbonate mixture in the presence of thepromoter, in order to accelerate a chemical reaction, is disclosed inU.S. Pat. No. 3,793,443.

However, U.S. Pat. No. 3,810,973 indicates that using, as a flux,compounds such as B₂ O₃, alkali metal borates or halides gives variableresults and accordingly, proposes to form a sodium chloride andpotassium chloride mixture in situ in order to promote the formation ofa ferrite.

U.S. Pat. No. 4,401,643 discloses a method for preparing barium ferriteswhereby a mixture of mixed barium and iron carbonate and of sodium saltis obtained by coprecipitation; then, this mixture is heated at atemperature higher than 680° C., but lower than the sodium salt meltingtemperature and a finely divided ferrite powder is obtained, which isseparated from the sodium salt.

Moreover, the technical literature discloses the preparation of numerousmetal oxides such as ZnO, SnO₂, In₂ O₃, MgO, TiO₂, by precipitating andcalcinating the corresponding hydroxide. In this literature, it is notmentioned or suggested to use a promoter or a flux during thecalcination step.

From the foregoing, it can be stated that it is still a problem toobtain metal oxides in the form of fine, homogeneous particlesexhibiting a determined morphology and appropriate conductiveproperties. The thermal treatment of oxides and the sintering itgenerates remain an obstacle in this regard.

The present invention is directed to an improved method for obtaining,in a simple and economical manner, a fine powder of metal oxideexhibiting electrically conductive properties.

The method consists in preparing a hydroxide precursor of a metal oxide,mixing this precursor with an inorganic compound having a melting pointlower than the crystallization temperature of said metal oxide and thensubmitting the resulting mixture to a thermal treatment at a hightemperature.

Preferably, the method consists in precipitating the hydroxideprecursor, separating and drying the resulting precipitate, mixing theprecipitate with an inorganic compound having a melting point lower thanthe crystallization temperature of said metal oxide and submitting it toa thermal treatment at a high temperature. The inorganic compound can beincorporated after or before drying the precipitate. Inorganic compoundscan be oxides such as boron oxide, or salts. Prefered compounds aresalts of alkali or earth alkali metals, and more especially halides orborates of alkali or earth alkali metals. The amount of inorganiccompound may vary widely depending on factors such as the nature of thehydroxide and the nature of the inorganic compound. The normally skilledman will determine in each case easily the appropriate amount to beused, but the inorganic compound to hydroxide weight ratio will becomprised generally between 0.5 and 20 and preferably between 1 and 10.

Using this inorganic compound allows one to perform the thermaltreatment in a more homogeneous medium, because the seed formation andthe particle growth are performed in a molten state. It results in lessclustering and, finer particles exhibiting a narrower size distribution.Taking this into account, the dimension and the size distribution of themetal oxide particles depend on the characteristics of the hydroxideprecipitate and on the drying method chosen.

The method of the present invention is particularly useful in order toprepare conductive oxides of transition metals such as tin, titanium,indium, molybdenum, aluminum, silicon, magnesium, barium, tungsten,vanadium, zinc or zirconium. Tin oxide is preferred. The methodaccording to the invention can serve to prepare simple oxides, mixedoxides or doped oxides. In the case of doped oxides, doping can beperformed with atoms of metals other than that of the oxide or withmetalloid atoms, for example, halogen atoms. Common doping agents aredisclosed in the literature and depend on the host oxide; for example,tin oxide SnO₂ can be doped with antimony, niobium or an halide. Theamount of doping agent, varying according to the oxide nature, thedoping agent nature and the applications considered, can range from 0,01to 20 mole %.

The oxide particles are obtained from a hydroxide; this hydroxide itselfcan be obtained by any of the techniques well-known in this respect,particularly by precipitation by means of alkali from salts in anaqueous, hydroalcoholic or alcoholic medium, or by alkoxide hydration orby decomposition of organometallic compounds. It is often desirable toremove the salt, at the end of the process; this can be simply performedby washing with water in the case of salts such as alkali metal halides.The resistivity of the resulting oxide particles widely varies dependingon the oxide nature, the possible doping but, considering applicationsusing the conductive character of these particles, the resistivity islower than 10⁸ ohm.cm and preferably, 10⁵ ohm.cm.

The thermal treatment conditions are well-known and disclosed in theliterature. These conditions must be controlled according to theirinfluence on the size and the morphology of the particles. It can bedesirable to perform, in some cases, the thermal treatment under aninert or reactive atmosphere, for example, under a reductive atmosphere.For the thermal treatment, the hydroxide can be sprayed in a colloidalform in an oven heated at a high temperature. It can also be carried outunder a vapor state or by evaporation under vacuum. The temperature ofthe treatment varies according to the oxide, but generally, it isranging from 400° to 1200° C., and preferably from 600° to 900° C.

Because of their fineness, the particles obtained by the methodaccording to the invention are particularly adapted to the preparationof thin layers or coating compositions exhibiting antistatic properties,for example, from dispersions in appropriate binders or directly frompowders.

EXAMPLE 1

40 g of tin chloride IV, SnCl₄, 5H₂ O and 0,94 g of antimony trichlorideare dissolved in 700 ml of ethanol. After dissolution, both alcoholicsolutions are mixed and this mixture is slowly poured into 750 ml ofwater at 90° C. in order to co-precipitate the IV tin and antimonyhydroxides. This precipitate is allowed to ripen for 2 hours, then isfiltered, washed and dried at 120° C.

A portion of this precipitate which is calcinated at 800° C. during 3hours, is picked up. A tin and antimony oxide powder is thus obtained.The characteristics of this powder are illustrated in the table below.

Another portion of the precipitate, mixed with KCl according to aKCl/precipitate weight ratio equal to 7.3, is picked up. This mixture issubmitted to a 3 hour thermal treatment at 800° C. The characteristicsof the resulting powder are illustrated in Table I below. The specificsurfaces are measured by the well known BET absorption technique. Theresistances are measured on oxide pellets compacted under a pressure of500 kg/cm².

                  TABLE I    ______________________________________    Chemical            Specific    Resistance    treatment  Salt     surface (m.sup.2 /g)                                    ohm · cm    ______________________________________    1-a  800° C.-3h                   --       17.2      <10    1-b  800° C.-3h                   KCl      51.8      <10    ______________________________________

The increase in the specific surface shows the sintering reductionbetween the particles during the thermal treatment, and thus, a betterdispersibility. These dispersions can be applied as homogeneous thinlayers exhibiting conductive and antistatic properties.

EXAMPLE 2

The procedure of example 1 is repeated, except that, afterprecipitation, the precipitate is separated and dried by spraying.

A portion of the precipitate is simply thermally treated, while anotherportion is mixed with KCl according to a KCl/precipitate weight ratioequal to 7.3, then thermally treated. Conditions of thermal treatmentare those specified in Table II below.

                  TABLE II    ______________________________________    Chemical            Specific    Resistance    treatment  Salt     surface (m.sup.2 /g)                                    ohm · cm    ______________________________________    2-a  700° C.-1h                   --       26.9       20    2-b  700° C.-1h                   KCl      72.3      130    ______________________________________

I claim:
 1. A method of preparing a fine tin oxide powder comprising thesteps of:(1) precipitating tin hydroxide; (2) mixing said tin hydroxidewith an inorganic compound having a melting point lower than thecrystallization temperature of tin oxide to form a mixture; and (3)subjecting said mixture to a high temperature thermal treatment at atemperature of from 400° C. to 1200° C. to form said fine tin oxidepowder.
 2. A method of preparing a fine tin oxide powder that haselectrically-conductive properties comprising the steps of:(1)co-precipitating tin hydroxide together with a doping agent to form aco-precipitate, (2) mixing said co-precipitate with an inorganiccompound having a melting point lower than the crystallizationtemperature of tin oxide to form a mixture, and (3) subjecting saidmixture to a high temperature thermal treatment at a temperature of from400° C. to 1200° C. to form said electrically-conductive fine tin oxidepowder.
 3. A method as claimed in claim 2, wherein said doping agent isantimony.
 4. A method as claimed in claim 2, wherein said inorganiccompound is a halide or borate of an alkali or earth alkali metal.
 5. Amethod as claimed in claim 2, wherein the weight ratio of said inorganiccompound to said tin hydroxide is between 0.5 and
 20. 6. A method asclaimed in claim 2, wherein the weight ratio of said inorganic compoundto said tin hydroxide is between 1 and
 10. 7. A method as claimed inclaim 2, wherein said doping agent is employed at a concentration offrom 0.01 to 20 mole percent.
 8. A method as claimed in claim 2, whereinsaid high temperature thermal treatment is at a temperature of from 600°C. to 900° C.
 9. A method of preparing a fine tin oxide powder that haselectrically-conductive properties comprising the steps of:(1)co-precipitating tin hydroxide and antimony hydroxide to form aco-precipitate; (2) mixing said co-precipitate with an inorganiccompound having a melting point lower than the crystallizationtemperature of tin oxide to form a mixture; and (3) subjecting saidmixture to a high temperature thermal treatment at a temperature of from600° C. to 900° C. to form said fine electrically-conductive tin oxidepowder.
 10. A method of preparing a fine tin oxide powder that haselectrically-conductive properties comprising the steps of:(1)co-precipitating tin hydroxide and antimony hydroxide to form aco-precipitate; (2) mixing said co-precipitate with potassium chlorideto form a mixture; and (3) subjecting said mixture to a high temperaturethermal treatment at a temperature of from 600° C. to 900° C. to formsaid fine electrically-conductive tin oxide powder.